The overall objective of this proposal is to determine the clinical utility of monitoring the circulating c-neu oncogene related protein (designated neu-related protein, NRP) in large groups of prospectively followed patients with breast cancer. Recent studies have demonstrated that over-expression of the c-neu/HER-2/erbB-B2 oncogene in primary breast cancers from patients with axillary nodal metastases correlates with a poor prognosis. Monoclonal antibodies (MAbs) prepared against the extra-cytoplasmic domain of the protein of the c-neu/HER-2/erb-B2 oncogene have been used to construct an ELISA that detects the cellular p185 neu product. A preliminary study with the c-neu ELISA suggested that 15% of patients with primary breast cancer and 25% of patients with metastatic breast cancer have elevated NRP levels and that serial NRP levels from selected patients correlate with disease course. The proposed investigations are designed to determine if NRP levels correlate with or, more importantly, predict clinical outcome of breast cancer patients. In a collaboration with the CALG over the last 3 years, we have collected over 1800 plasma samples from 250 patients with metastatic breast cancer who participated in chemotherapy and hormone treatment protocols. We are currently collecting samples from patients with newly diagnosed primary breast cancer who are participating in adjuvant trials. We have already accrued approximately 250 of an expected 750 patients with stage II cancer and anticipate that approximately 100 patients will be accrued to a newly activated stage III protocol. Anti-neu MAbs will be utilized to immunoprecipitate NRP from plasma samples containing very high levels in order to investigate the association of circulating NRP with the cellular oncogene product (Specific Aim 1). Plasma samples from patients with metastatic disease will be assayed with the c-neu ELISA to determine a) the distribution of elevated NRP levels; b) the relationship of serial NRP levels with clinical course c) the prognostic value of initial NRP levels, in terms of response to therapy, time to progression, and overall survival; and d) the relationship between circulating NRP levels and primary tissue c- neu expression as monitored by immunoperoxidase staining (Specific Aim 2). Similarly, in patients with newly diagnosed stage II and III breast cancer, we will determine a) the prevalence and prognostic significance of initially elevated NRP levels; b) the relationship of initial NRP levels with response to therapy for stage III patients; c) the predictive value for relapse of a rising NRP level after primary and adjuvant therapy; and d) the relationship between circulating NRP levels and primary tissue c-neu amplification (Southern Blot analysis) and expression (c-neu ELISA and immunoperoxidase staining) (Specific Aim 3).